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Chapter 2
Helical Foundation Systems
CHAPTER 2
HELICAL FOUNDATION SYSTEMS
requirements detailed in FHWA Geotechnical
Engineering Circular No. 7 (Lazarte, Elias et al.
2003). Several computer programs are available
for design of soil nail walls, with the more
common programs being SNAIL (CALTRANS
1999) and GoldNail (Golder 1996).
SNAIL is a Windows-based program developed
by the California Department of Transportation
(CALTRANS) and is available free to the public.
SNAIL is a two dimensional limit equilibrium
program that uses force equilibrium exclusively.
Either metric or English units can be used during
the design process. Soil nail reinforcement
inputs include location, diameter, inclination
angle, vertical and horizontal spacing, cross
sectional area, yield strength and tensile
strength. The soil parameter inputs include
soil unit weight, cohesion, friction angle, bond
strength and the bond strength reduction factor.
The soil strength parameters are modeled
with the conventional linear Mohr-Coulomb
envelope. The only data entered for the wall
design is the face punching shear, therefore an
initial wall facing design must be used for the
trial runs. The program allows for consideration
of up to seven soil layers and provides inputs
for two uniform vertical surcharge loads and
an internal or external point load. The program
output provides the global factor of safety, an
estimated location of the failure plane and the
tensile forces for each nail for each of the 10
most critical failure surfaces analyzed.
GoldNail is a Windows-based proprietary
program developed by Golder Associates which
satisfies both moment and force equilibrium.
The program can work in one of three modes;
design, factor of safety and nail service load.
The program allows factored strengths for Load
and Resistance Factor Design (LRFD). The soil
strength parameters can be modeled with the
conventional linear Mohr-Coulomb envelope or
using a bi-linear strength envelope. Up to 13
soil layers can be modeled with more complex
geometry capability than SNAIL. The program
can only model a circular failure surface which
must pass at the toe of the wall or above the
toe. This limits the ability to evaluate sliding
and bearing capacity failure modes. Data input
variables and output reports are generally similar
to SNAIL.
FHWA Geotechnical Engineering Circular No. 7
(Lazarte, Elias et al. 2003) provides design tables
and charts that can be used for preliminary
estimation of the wall design. The tables and
charts were developed using SNAIL simulations
and include the following assumptions:
• The soil is homogenous (only one soil type and
strength parameter)
• There are no surcharge loads or sloped
backfill conditions
• There are no seismic forces/loads
• The soil nails are of uniform length, spacing
and inclination for each row
• There is no groundwater present
There should always be a final design
prior to construction activities which take
into consideration any deviations from the
assumptions listed above and determination of
the Limit States described in Section 2.9.2.2.
The design of helical soil nails should
be completed by experienced design
professionals. Installation of FSI helical
soil nails shall be by certified FSI Installing
Contractors trained specifically for helical soil
nail installations. FSI recommends that thewall
design follow the general guidelines detailed in
the FHWA Geotechnical Engineering Circular
No. 7 (Lazarte, Elias et al. 2003).
Preliminary design recommendations are
available to FSI Installing Contractors to
assist with costing of helical soil nail wall
projects. However, the final design must be
completed and/or approved by the engineer
of record.